1. Field of the Invention
The present invention relates to an optical pickup device capable of recording label information onto a label layer of a signal recording medium, and a focus control method therefor.
2. Description of the Related Art
DVD drives for recording and reproducing data to and from a DVD (digital versatile disc) are widely available. In most cases, a DVD drive incorporates an optical pickup device that can ensure compatibility with a CD (compact disc). Such an optical pickup device has two types of sources of laser light that emit two types of laser light having different wavelengths, such as two types of laser diodes, one of which emits a beam of laser having a suitable wavelength for the recording density of DVDs, and the other of which emits a beam of laser having a suitable wavelength for the recording density of CDs. By switching between the sources of light in accordance with the recording density of a disc in use, a single optical pickup device can be compatible with both DVDs and CDs having different recording densities.
When measured from a surface of a transparent substrate to a signal layer, a DVD has a thickness of 0.6 mm, whereas a CD has a thickness of 1.1 mm. In other words, the thicknesses of the transparent substrates are significantly different at a ratio of approximately 1 to 2. In a DVD and CD compatible optical pickup device, different NAs (numerical apertures) are required of an objective lens to achieve suitable optical characteristics for the respective types of discs. Therefore, when compatibility with both DVDs and CDs is provided using a single objective lens, the optical pickup device employs a two-focal-length objective lens configured to provide suitable NAs for the respective types of discs.
In order to identify the content of information recorded on a disc after the recording, the disc may be labeled with label information indicating the content of the recorded information.
Typically, such labeling of a disc with label information is provided by printing or handwriting the label information on a label surface of the disc. Recently, a technique that uses laser light applied from an optical pickup device to record the label information in the form of a visible image on a heat-sensitive label layer formed in the disc has been developed.
Currently, the recording of a visible image that represents label information is performed after completion of recording of main information that is to be recorded on a signal layer of a disc. However, if label information can be automatically recorded simultaneously with the recording of main information, the overall recording time can be shortened. Further, it is possible to eliminate the inconveniences associated with the operational steps required for recording of label information. Further, it is also possible to record main information and label information without creating a mismatch therebetween.